1. Field of the Invention
The present invention relates to exposure-control devices for use in photographic apparatus, and more particularly to shutter-control devices for automatically establishing exposure time in accordance with the level of scene illumination.
2. Description of the Prior Art
Two-bladed camera shutters are well-known, and many include closing blades which are spring-biased toward a closed position and are held in an open position by magnetic force. Upon reduction of the magnetic attractive force applied to the blade, the spring moves the blade to an aperture-closing position to terminate exposure.
U.S. Pat. No. 3,205,799 and British Pat. No. 1,365,560 show different approaches for releasing the magnetically held closing blade. In the U.S. patent, an electromagnet holds the closing blade when energized and, upon a decrease in the electromagnet coil current, releases the blade. The British patent provides a permanent magnet for retaining the closing blade in an open position. To terminate an exposure, an electromagnet coil is energized to overcome the magnetic attraction between the permanent magnet and the closing blade.
It would appear that either of these prior-art camera designs would work satisfactorily in cameras of ordinary exposure intervals. However, because current does not change instantaneously in an electromagnetic coil, exposure control apparatus of the type described above will have a response time related to the relevant time constants. In some applications, such as use with high speed shutters, the response time may render such apparatus incapable of providing accurate exposure control. For that reason, we have provided a fast-acting piezoelectric motor for slightly separating the shutter blade from the magnet holding it open such that the magnetic force is reduced sufficiently to be overcome by a spring force urging the closing blade to an aperture-closing position.
Apparatus in which a flexing-type piezoelectric element is used to terminate an exposure interval is disclosed in commonly assigned, copending U.S. patent applications Ser. No. 816,776, entitled PIEZOELECTRIC CAMERA SHUTTER, filed on July 18, 1977 and Ser. No. 816,835, entitled TIMED PIEZOELECTRIC SHUTTER CONTROL FOR CAMERAS, filed on July 18, 1977. In the latter application, a shutter-control device is disclosed in which a closing shutter blade mechanically latches in an open position during an exposure and is unlatched when a voltage is suddenly applied to a piezoelectric Bimorph element to rapidly change its shape. The Bimorph element flexes rapidly to "fling" the latch open to release the closing blade so that the shutter closes.
Although such a mechanism is highly accurate with respect to shutter speed, the manufacturing and assembly tolerances of the closing blade latch are important to successful operation so that the latch securely holds the blade yet quickly releases the blade when flung open. Further, such latches have exhibited problems associated with wear of parts which slide against one another. By the present invention, we have provided a more easily assembled and more durable device for holding the closing blade and for releasing it upon sudden application of an electrical voltage.